Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

The results show that children treated with antibiotics between birth and 5 months of age were 22 percent more likely to be overweight at age 3.

However, exactly when children were given antibiotics mattered: children treated with the drugs between the time they were 6 months and 14 months old did not have significantly higher body mass at age 3 than children who did not receive antibiotics during that time, the researchers said.

This indicates that the first 6 months of life may be "a window of special vulnerability to exposure," the researchers wrote in the Aug. 21 issue of the International Journal of Obesity.

"We typically consider obesity an epidemic grounded in unhealthy diet and exercise, yet increasingly studies suggest it's more complicated," study researcher Dr. Leonardo Trasande, associate professor of pediatrics and environmental medicine at New York University School of Medicine, said in a statement. "Microbes in our intestines may play critical roles in how we absorb calories, and exposure to antibiotics, especially early in life, may kill off healthy bacteria that…would otherwise keep us lean," Trasande said.

The researchers noted the study found an association, and not a cause-effect link, and that future studies are needed to determine whether taking antibiotics early in life plays a role in causing obesity.

In recent years there has been a growing concern about the overuse of antibiotics, especially in children. Preliminary studies suggest that changes in the composition of gut bacteria are linked to obesity, inflammatory bowel disease, asthma and other conditions.

In the new study, Trasande and colleagues evaluated the use of antibiotics in 11,500 children born in the United Kingdom between 1991 and 1992.

It's possible that the way antibiotics are administered may influence their effect on gut bacteria composition and development, the researchers said. Before age 6 months, antibiotics are given intravenously, often for infant sepsis.